1. Field of the Invention
The present invention relates to a device and method for controlling a power converting device for performing pulse width modulation (PWM) on an output from a direct current (DC) power supply and thereby producing an output of alternating current (AC) power, and is concerned with a technique for preventing the occurrence of a malfunction due to a change in the frequency of a pulse width modulation carrier.
2. Description of the Related Art
Apparatuses (such as a current control stepping motor) driven by a pulse width modulated signal are configured to control its operation by changing a duty ratio of a current waveform fed through the apparatus (or by performing pulse width modulation (hereinafter referred to simply as “PWM”)). This PWM-based controlling apparatus produces switching noise of a fundamental control frequency and higher harmonics thereof, because of doing switching to turn on or off a load driving current with a train of pulse width modulated pulses. For example when the apparatus is mounted on a vehicle, the switching noise can possibly have an influence on hearing of a radio mounted on the identical vehicle and render it difficult to hear the radio or cause the radio to produce a harsh grating noise, or have an adverse effect on operation of other vehicle-mounted digital instruments.
Japanese Unexamined Patent Application Publication No. 7(1995)-99795 discloses a stepping motor control device designed for the purpose of reducing the above-mentioned noise. The device subjects the drive current pulse train (or a control clock) previously subjected to the PWM to control the current control stepping motor to additional frequency modulation with a sinusoidal wave having a lower frequency than a control clock frequency. This effects the spread of spectrum components of the noise over a desired frequency band, thus reducing the influence of the noise upon the vehicle-mounted apparatus or instruments.
Advances in digital processors and digital circuit techniques make it possible to use a central processing unit (CPU) or the like to execute operations for calculating a PWM pattern, and make it possible to use a digital timer and a digital comparator circuit to generate the PWM pattern with precision, thus facilitating PWM pattern generation even if the pattern is complicated.
As shown in FIG. 1 of Japanese Unexamined Patent Application Publication No. 7(1995)-99795, a power converting device is used for current control, which is configured to effect a periodic change in the frequency of a pulse width modulation carrier in order to attenuate the noise components. However, in such a power converting device, a change in a current calculation period according to the change in the carrier frequency (or period) takes place in the device that performs current sampling, control calculations or the like in synchronization with a PWM generation period. Due to this, current control characteristics change according to whether the carrier period is short or long. Consequently, periodic variations caused by the change in the frequency of the pulse width modulation carrier occur as variations in an output current waveform, as shown in FIG. 18 to be discussed later (in which Tm denotes a change period). The occurrence of the variations in the output current waveform from the power converting device can possibly lead to variations in operation of the load driven by the output from the power converting device (e.g., variations in torque of the motor).